A large number of microorganisms have been found to accumulate polyesters, as energy storage compounds, in the cells thereof. A typical example thereof is poly-3-hydroxybutyrate [hereinafter simply referred to as P(3HB)], which is a homopolymer containing a monomer unit (3HB) represented by the following Formula: ##STR1##
P(3HB) is a so-called biodegradable plastic, which undergoes biological degradation in natural environments; however, when viewed as a polymer material, it is insufficient for practical use because it is highly crystalline, hard and brittle.
As a means for overcoming these drawbacks, it has been proposed to incorporate a monomer unit which is structurally different from 3HB to compose the polyester. The methods based on this concept can be roughly divided into two groups as follows.
(1) According to Japanese Patent Laid-Open Nos. 150393/1982, 69225/1983, 269989/1988, 48821/1989 and 156320/1989, copolymer P(3HB-CO-3HV), containing 3-hydroxyvalerate (simply referred to as 3HV) and 3HB, is obtained by culturing Alcaligenes eutrophus, a microorganism which essentially produces P(3HB), from a carboxylic acid having an odd number of carbon atoms, such as propionic acid or valeric acid, as a carbon source. Similarly, it is reported that copolymer P(3HB-CO-4HB), containing 4-hydroxybutyrate (simply referred to as 4HB) and 3HB, is obtained from 4-hydroxybutyric acid or .gamma.-butyrolactone, as a carbon source. ##STR2##
(2) According to Japanese Patent Laid-Open No. 226291/1988, it is reported that copolymer P(3HA), having 3-hydroxyalkanoate (simply referred to as 3HA) having 6 to 12 carbon atoms can be biosynthesized by Pseudomonas oleovorans ATCC29347, a hydrocarbon-utilizing bacterium, from alkanes as a carbon source. Here, to provide a clear representation of the relationship between each monomer unit structure and carbon number in 3HA, this monomer unit is referred to as a C.sub.x unit. ##STR3##
According to the above-mentioned patent publications, 3HB is a C.sub.4 unit and 3HV is a C.sub.5 unit; Pseudomonas oleovorans is capable of intracellularly synthesizing and accumulating copolymers containing C.sub.6 to C.sub.12 units.
Also, "Applied and Environmental Microbiology, 1988, pages 1977-1982" states that the carbon source alkanes should have at least 6 carbon atoms in order for Pseudomonas oleovorans to synthesize a polyester, and that units exceeding C.sub.12 is not synthesized even if an alkane having a carbon number of 12 or more are added.
As stated above, two types of copolymer have been proposed. The copolymers described in (1) have a small number of methylene groups in the side chain thereof, and they are physically plastic-like polymers. The copolymers described in (2) have a large number of methylene groups in the side chain thereof, and they are physically gel-like polymers. However, with respect to the copolymers described in (1) above, the costs of cultivation are inevitably high because starting materials for 3HV, 4HB and other copolymer components must be separately added, in addition to the major carbon source as a starting material for 3HB. For this reason, the discovery of strains which synthesize copolymers from cheap starting materials and the establishment of conditions for their culture have been of concern.
The present inventors have conducted investigations in a search for microorganisms which utilize long-chain fatty acids and naturally occurring oils and fats to biologically synthesize and accumulate a polyester in the cells thereof, and have found strains which accumulate plastic-like two- to four-component copolymers described in (1) above having a small number of methylene groups in the side chain thereof. The present inventors have made further investigations based on this finding, and thus developed the present invention.